Gypsum products are commonly used in industrial and building products, particularly gypsum panels or wallboard, due to their ready availability and low cost. By dehydrating and rehydrating gypsum, also known as calcium sulfate dihydrate or landplaster, it can be formed into articles of many useful shapes. Gypsum products are fire retardant, at least in part due to the association of two water molecules with each calcium sulfate molecule. Without reinforcing materials, formed, dry gypsum is relatively brittle, and cannot support heavy loads or sustain impacts of significant force.
The addition of host materials, such as wood or paper fibers, is known to improve the tensile and flexural strength of wallboard. However, the reduction in calcium sulfate dihydrate and the addition of flammable material reduces the fire retardency of the finished product and is more costly.
Attempts to make particle reinforced gypsum products without paper facings, such as fiberboard, have not yielded the improvements in strength that were expected. While not wishing to be bound by theory, it is believed that the lack of bonding between the gypsum particles and the reinforcing particles allows separation at the interface, and the inability of the composite to form a sturdy board.
In U.S. Pat. No. 5,320,677 to Baig, it was disclosed that a strong composite product could be made by mixing calcium sulfate dihydrate with wood particles prior to calcination of the gypsum. After a dilute slurry is formed of gypsum and particles, it is heated under pressure to at least 140° C. to convert the calcium sulfate dihydrate into the alpha form of calcium sulfate hemihydrate, also known as stucco or plaster of Paris. The alpha form used in this process is characterized by the formation of elongated, acicular crystals. As the crystals form, some of them will form within pores, cavities and other imperfections in the particle surface. When the slurry cools, the stucco rehydrates, forming an interlocking matrix of gypsum crystals and reinforcing particles.
There has also been a considerable amount of work on improving the water resistance of gypsum products. Hydrocarbons, including wax and asphalt are suggested to reduce water uptake in U.S. Pat. No. 2,198,776. Inorganic materials including metallic soaps and silicones have been used as coatings on the surface of the finished gypsum product. Water-resistance of products using wax, for example, is not very reproducible. Results from the current wax system used at the manufacturing facility is variable, so the wax is overused (at a higher overall cost) to safely keep the immersion values within an acceptable level.
The prior art teaches that siloxanes are useful when added to the slurry in the form of an emulsion. A siloxane emulsion was added to fiberboard products in U.S. Pat. No. 5,817,262 to Englert. The stable emulsion was added to the aqueous slurry of gypsum after calcinations then rehydrated. A filter cake was formed by dewatering the slurry, pressing the filter cake over porous surfaces. The cake was then allowed to set, then dried in a conventional kiln. However, this board lacked the strength that was expected.
It is an object of the present invention to provide a fiberboard panel having improved water resistance combined with higher strength.